With increase in depth of oil wells and gas wells (hereinafter, the oil wells and the gas wells are collectively and simply referred to as “oil wells”), there is a need for increase in strength of oil well steel pipes. Conventionally, 80 ksi grade (yield stress of 80 to 95 ksi, that is, 551 to 654 MPa) and 95 ksi grade (yield stress of 95 to 110 ksi, that is, 654 to 758 MPa) oil-well steel pipe have widely been used. However, recently, 110 ksi grade (yield stress of 110 to 125 ksi, that is, 758 to 862 MPa) oil well steel pipes are put into use.
Many of the recently-developed deep wells contain hydrogen sulfide, which is corrosive. Thus, oil well steel pipes are required to have not only high strength but also sulfide stress cracking resistance (hereinafter referred to as SSC resistance).
As a measure to improve the SSC resistance of a conventional 95 to 110 ksi grade oil well steel pipe, known methods include cleaning the steel and making the steel structure finer. For example, Japanese Patent Application Publication No. 62-253720 proposes the method for improving the SSC resistance by reducing impurity elements such as Mn and P. Japanese Patent Application Publication No. 59-232220 proposes the method for improving the SSC resistance by performing quenching twice for grain refining.
In response to the need for increase in strength of the oil well steel pipes, recently, 125 ksi grade (yield stress of 862 to 965 MPa) oil well steel pipes are put into use. Furthermore, there is an increasing need for oil well steel pipes having yield strength of 140 ksi (yield stress of 965 MPa) or more.
Sulfide stress cracking (SSC) is more liable to occur as the strength is higher. Therefore, oil well steel pipes of 140 ksi or more need to have further improved SSC resistance, compared to the conventional oil-well steel pipes of the 95 ksi class, the 110 ksi grade and the 125 ksi grade.
Japanese Patent Application Publication Nos. 6-322478, 8-311551, 11-335731, 2000-178682, 2000-256783, 2000-297344, 2000-119798, 2005-350754 and 2006-265657 each propose a measure to improve the SSC resistance of a steel for a high-strength oil well pipe.
Japanese Patent Application Publication No. 6-322478 proposes the method for improving the SSC resistance of a 125 ksi grade steel product by providing a finer steel structure by heat treatment using induction heating. Japanese Patent Application Publication No. 8-311551 proposes the method for improving the SSC resistance of a steel pipe of the 110 to 140 ksi grades by enhancing the hardenability using the direct quenching process and increasing the tempering temperature. Japanese Patent Application Publication No. 11-335731 proposes the method for improving the SSC resistance of a low alloy steel of the 110 to 140 ksi grades by making adjustment to provide optimum alloy chemical composition. Japanese Patent Application Publication Nos. 2000-178682, 2000-256783 and 2000-297344 each propose the method for improving the SSC resistance of a steel for a low alloy oil well pipe of the 110 to 140 ksi grades by controlling the shapes of carbides. Japanese Patent Application Publication No. 2000-119798 proposes the method for delaying a time of occurrence of SSC in a steel product of the 110 to 125 ksi grades by precipitation of a large amount of fine V carbides. Japanese Patent Application Publication No. 2005-350754 proposes the method for improving the SSC resistance of an oil well steel pipe of the 125 ksi grade or more by controlling the dislocation density and the hydrogen diffusion coefficient to desired values. Japanese Patent Application Publication No. 2006-265657 proposes the method for improving the SSC resistance of a steel for an oil well pipe of the 125 ksi grade or more by production of a single-phase bainite structure, which is provided by containing a large C content in the steel, and during water cooling, stopping the water cooling at 400 to 600° C. and performing isothermal transformation heat treatment (austemper treatment) at 400 to 600° C.